The present invention provides an RF amplifier gate bias circuit that is appropriate for use in a wide range of frequencies and applications having no particular bounds and including KHz to GHz, including in the L, S, and C bands. The S band ranges from 2 to 4 GHz and is part of the microwave band of the electromagnetic spectrum used in weather radar, surface ship radar, and communications satellites applications. The L band, referred to as the IEEE L band, is a portion of the microwave band of the electromagnetic spectrum ranging from 1 to 2 GHz. The L band is used in communications, digital audio broadcast, satellite communications, telecommunications, military, telemetry as well as other applications. For instance, the Global Positioning System (GPS) utilizes carriers in the L band. Uses for IEEE C-band frequencies, which extend from 4 to 8 GHz, include satellite communications, weather radar, and military applications.
Laterally Diffused Field Effect Transistor (LDFET), also referred to as Laterally Diffused Metal-Oxide Semiconductor (LDMOS), type Radio Frequency (RF) devices have several advantages over bipolar transistors such as higher gain, higher efficiency, and wider dynamic range of output power. LDFETs also have a major disadvantage in that the gate bias voltage (Vg) required to set the quiescent current (Id) drifts over temperature, time, input drive, and frequency, as well as from device to device variations. Considerable effort has been expended by the various manufacturers of these devices to lessen this undesirable effect, but no one has fully solved the problem.
Exemplary uses of the RF amplifier gate bias circuit of the present invention are transmission applications, including transmitters, receivers, and power amplifiers.
What is needed is a solution to address the various undesirable operational side effects associated with use of LDFET, GaNFET, GaAsFET, JFET and other such transistors to more fully and efficiently take advantage and utilize their beneficial properties and to expand the acceptable use of such devices in a wider range of RF applications.
In addition, N-Channel depletion device-based amplifiers operate with the negative characteristic of N-channel depletion mode devices that requre a negative gate voltage and gate-drain bias sequencing for proper operation. With any N-Channel depletion device, such as GaAs FET, GaN FET, or N-channel silicon junction FET, it is essential that the negative gate voltage arrives before the drain voltage otherwise the drain to source resistance is a very low value which will essentially short out the input power and likely cause damage to several circuit components including the depletion device. Existing approaches to sequencing for GaN devices, e.g., test fixture set up for fire-up and shut-down sequencing, are cumbersome and are external to the device, e.g., amplifier. For example, supplying a negative voltage on a test fixture or lab bench is typically accomplished with an external supply having negative voltage generation capability or by switching the leads between the ground node and the positive voltage node. In an application circuit the negative voltage comes from a regulator or a negative voltage generator. The goal in bias sequencing the device is to avoid areas that are sensitive to potential instability of the device, e.g., the area where VDS drain to source is low and IDS drain to source is high. What is needed is an improved sequencer for use in RF amplifiers employing N-channel depletion mode devices that is internal to the amplifier circuit or device and that is flexible in accommodating a variety of such devices having differing attributes.
Applications for the invention include two-way private radio communication, broadband amplifiers, cellular infrastructure, test instrumentation, and Class A, AB, Linear amplifiers suitable for OFDM, W-CDMA, EDGE, CDMA waveforms.
As discussed above, temperature compensation is another aspect to circuit integrity and this has further relevance to bias sequencing and to adequately maintain the bias of the device for consistent performance over temperature. The quiescent current of a GaN HEMT device is primarily a function of temperature and VGS. What is needed is a bias circuit with temperature compensation that can maintain consistent operational performance over a prescribed range of temperature fluctuation, e.g., −50 to 100 degrees Celsius.